The present invention relates to an information identification (ID) system capable of managing a plurality of commodities each having mounted thereon a response unit, by reading the unique information held in the response unit, and the control unit and response units for realizing the system.
A technique for reading the ID data of a radio tag attached to a commodity or the like is known as disclosed in U.S. Pat. No. 5,537,105, for example. In this technique, each radio tag transmits a response signal including ID data to an inquiry signal from a questioner. In the process, each radio tag generates a predetermined random number and delays the timing of transmitting a response signal based on the value of the random number. As a result, the collision probability of the signals sent out by a plurality of radio tags is reduced.
In spite of the fact that the transmission timing of each radio tag is differentiated based on random numbers as described above, the collision probability increases if the bit length of the random numbers is small. In fact, an indefinite factor exists by which collision is repeated endlessly. An increased bit length, on the other hand, though capable of suppressing the collision probability, increases the waiting time, resulting in an increased overall read time. Further, in the case where the number of objects to be read is small, the waiting time occupies a considerable proportion of the whole time during which only a small number of signals are transmitted, thereby leading to a very low efficiency.